"Capitalism" is just a made-up word meaning the natural state of human affairs, in which people own property, do work, buy, sell, and trade with one another. The word only became popular in the past couple centuries as socialists and fascists tried to present capitalism as "just another -ism", just another "option" that society might choose or not choose.
The falsehood in the claim that capitalism is an "ism" can be demonstrated by the fact that no nation or culture has ever "imposed" capitalism upon its people. All forms of socialism, fascism, redistributism, Keynsianism, etc, are imposed by government taking deliberate action against the natural economic choices of their people. What you call "capitalism" is just the absence of such interventions. Capitalism is just a word to describe the natural economic life of man.
He is arguing that the inequality capitalism generates is unsustainable.